To date, modern power semiconductor diodes such as high-voltage P-I-N diodes, as well as power transistors such as power MOSFETs and Insulated Gate Bipolar Transistors (IGBT), have been typically fabricated with silicon (Si) semiconductor materials. More recently, silicon carbide (SiC) power devices have been researched due to their superior properties. III-Nitride (III-N) semiconductor devices are now emerging as an attractive candidate to carry large currents and support high voltages, and provide very low on resistance, high voltage device operation, and fast switching times. As used herein, the terms III-N or III-Nitride materials, layers, devices, etc., refer to a material or device comprised of a compound semiconductor material according to the stoichiometric formula AlxInyGazN, where x+y+z is about 1.
Examples of III-N high electron mobility transistors (HEMTs) and III-N diodes, respectively, of the prior art are shown in FIGS. 1 and 2. The III-N HEMT of FIG. 1 includes a substrate 10, a III-N channel layer 11, such as a layer of GaN, atop the substrate, and a III-N barrier layer 12, such as a layer of AlxGa1-xN, atop the channel layer. A two-dimensional electron gas (2DEG) channel 19 is induced in the channel layer 11 near the interface between the channel layer 11 and the barrier layer 12. Source and drain contacts 14 and 15, respectively, form ohmic contacts to the 2DEG channel. Gate contact 16 modulates the portion of the 2DEG in the gate region, i.e., directly beneath gate contact 16. The III-N diode of FIG. 2 includes similar III-N material layers to those of the III-N HEMT of FIG. 1. However, the III-N diode of FIG. 2 only includes two contacts, an anode contact 27 and a cathode contact 28. The anode contact 27 is formed on the III-N barrier layer 12, and the cathode contact 28 is a single contact which contacts the 2DEG 19. The anode contact 27 is a Schottky contact, and the single cathode contact 28 is an ohmic contact. In FIG. 2, while there appears to be two cathode contacts, the two contacts are in fact electrically connected so as to form a single cathode contact 28.
Field plates are commonly used in III-N devices to shape the electric field in the high-field region of the device in such a way that reduces the peak electric field and increases the device breakdown voltage, thereby allowing for higher voltage operation. An example of a field plated III-N HEMT of the prior art is shown in FIG. 3. In addition to the layers included in the device of FIG. 1, the device in FIG. 3 includes a field plate 18 which is connected to gate 16, and an insulator layer 13, such as a layer of SiN, is between the field plate and the III-N barrier layer 12. Field plate 18 can include or be formed of the same material as gate 16. Insulator layer 13 can act as a surface passivation layer, preventing or suppressing voltage fluctuations at the surface of the III-N material adjacent to insulator layer 13.
Slant field plates have been shown to be particularly effective in reducing the peak electric field and increasing the breakdown voltage in III-N devices. A prior art III-N device similar to that of FIG. 3, but with a slant field plate 24, is shown in FIG. 4. In this device, gate 16 and slant field plate 24 are formed of a single electrode 29. Insulator layer 23, which can be SiN, is an electrode-defining layer that contains a recess which defines at least in part the shape of electrode 29. Electrode-defining layer 23 can also act as a surface passivation layer, preventing or suppressing voltage fluctuations at the surface of the III-N material adjacent to electrode-defining layer 23. The gate 16 and slant field plate 24 in this device can be formed by first depositing electrode-defining layer 23 over the entire surface of III-N barrier layer 12, then etching a recess through the electrode-defining layer 23 in the region containing gate 16, the recess including a slanted sidewall 25, and finally depositing electrode 29 at least in the recess and over the slanted sidewall 25. Similar slant field plate structures can be formed in III-N diodes. For example, a III-N diode similar to that of FIG. 2 can also include a slant field plate connected to the anode contact 27.
Slant field plates, such as field plate 24 in FIG. 4, tend to spread the electric fields in the device over a larger volume as compared to conventional field plates, such as field plate 18 in FIG. 3, which do not include a slanted portion. Hence, slant field plates tend to be more effective at reducing the peak electric field in the underlying device, thereby allowing for larger operating and breakdown voltages.
While slant field plates are desirable for many applications, they can be difficult to fabricate reproducibly. Field plate structures that can provide adequate suppression of peak electric fields and can be fabricated reproducibly are therefore desirable.